1. Field of the Invention
This invention relates to a valve, wherein the fluid pressure on the upstream side of the valve seat is decreased when the gate is moved to an open position.
2. Description of the Prior Art
As is well known, a valve, such as a gate valve or a ball valve, includes annular valve seats fitted in valve seat retainers provided in the peripheral portions of fluid passages in the valve body, respectively. A valve seat which is generally made of rubber, bulges out through an opening in the valve seat retainer due to the pressure of the fluid on the upstream side of the valve seat retainer and is thereby forced into close contact with the wall surface of the gate, thus providing the sealing therebetween when the gate is moved to a closed position. The valve is such that in order to shift the valve between the open and closed positions, the gate is continuously slidingly moved, with its wall surface maintained in close contact with the wall of the valve seat, rather than the wall surface of the gate being instantaneously separated from the valve seat. With such a valve, a shortcoming due to structural factors arises when at the initial stage during which time the gate is being moved to an open position, part of the valve seat on the upstream side, which has become disengaged from the wall surface of the gate, is undesirably forced from the opening in the valve seat retainer to project to the exterior thereof. This phenomenon will be described in more detail by the following with reference to FIGS. 1 through 5.
FIG. 1 shows an example of a gate valve. A valve body 1 has horizontal fluid communication holes 2a and 2b running transversely thereof, and a vertical guide passage 4 for a gate 3. Within guide passage 4 is fitted a gate 3, which is adapted to move vertically into open and closed positions by the upward and downward motion of a stem 5 connected to the top of gate 3. Gate 3 is a flat plate having a given thickness as shown in FIG. 2, which has a fluid blocking portion 3a in the upper portion thereof and an opening portion 3b in the lower portion.
In the peripheral portions of fluid communication holes 2a and 2b in valve body 1 are provided seal rings 6a and 6b which slidingly contact the opposite walls of gate 3, respectively. Each seal ring, 6a and 6b, includes a resilient valve seat 7, a valve seat retainer 9 for holding the valve seat, and an annular sealing member 8 provided in the fitting portion between the seat retainer 9 and valve body 1. The diameter of annular sealing member 8 is selected so as to provide the necessary self-tightening force such that each seal ring, 6a and 6b, is brought into close contact with gate 3 by the fluid pressure to provide the sealing therebetween. Seal rings 6a and 6b are usually biased inwardly from the valve body, namely, towards gate 3, under the force of coil springs 10.
Operation of the seal ring will now be described. For explanatory simplicity, the description will be limited to the upstream side (seal ring 6a). When gate 3 assumes the closed position, valve seat 7 is maintained in close contact with valve body 3 by virtue of the self-tightening force by the fluid pressure, thus providing the sealing therebetween.
The opening and closing of gate 3 is achieved by upward and downward motion thereof. When blocking portion 3a of gate 3 faces fluid communication holes 2a and 2b in valve body 1, communication of fluid is interrupted; whereas when opening portion 3b of gate 3 faces fluid communication holes 2a and 2b, fluid is allowed to flow, for example, from fluid communication hole 2a by way of opening portion 3b of gate 3 into fluid communication hole 2b.
Valve seat 7 acts in the manner shown in FIG. 3, which shows gate 3 in the closed position. Fluid within fluid communication hole 2a on the upstream side is introduced, by way of a small gap between seat retainer 9 and blocking portion 3a of gate 3 into a groove 11 in valve seat retainer 9, thereby pressing valve seat 7 fitted in groove 11 under a pressure of P.sub.1. Resilient valve seat 7, under fluid pressure from the upstream side, tends to be swelled out from opening 13 of groove 11 in the valve seat retainer 9 into close contact with the wall surface of gate 3 as well as the bottom surface of groove 11. The surface-bearing pressure P.sub.2 acting on the face surface of gate 3 is higher than pressure P.sub.1 of the fluid, so as to provide perfect sealing therebetween.
When the gate 3 is in the closed position, pressure of the fluid on the upstream side is exerted on valve seat 7, such that a shortcoming arises when gate 3 is moved from the closed position to the open position. Such shortcoming arises upon the displacement of blocking portion 3a of valve body 3 with respect to valve seat 7. Stated otherwise, when gate 3 is moved upward, the relationship of the gate 3 with valve seat 7 as shown in FIG. 4, is established. Thus, when the bottom end of blocking portion 3a of gate 3 is separated from valve seat 7, valve seat 7 in turn intersects the top end of opening portion 3b of gate 3. Valve seat 7 is thus urged toward opening 13 by pressure P.sub.1 of the fluid within groove 11 with the result that the valve seat is eventually forced out of the groove 11 in valve seat retainer 9 and forced into opening portion 3b of gate 3, as shown in FIG. 3, thereby impairing the valve function.
The prior art has thereby attempted to solve this problem by such means as: (1) making valve seat 7 of a hard material so as to impart an increased resistance to deformation; and (2) increasing the size of opening 13 of groove 11 in valve seat retainer 9. These means have disadvantages in the former prior art solution, when the working fluid is low in pressure, the valve seat fails to swell so as to closely contact the wall surface of the gate, resulting in the lowered sealing performance, and in the latter prior art solution, reducing the size of the opening limits the extent of valve seat swelling with the result being failure to provide perfect sealing when the working fluid pressure is low. Furthermore, because of the limited tensile strength of resilient materials such valve seats cannot be utilized under high tension.